The goal of this project is to develop highly effective cancer-targeting agents for cancer therapy. Radioimmunotherapy (RIT) has proven to be useful in cancer patients. However, the dose-limiting toxicities of RIT are due to excessive radiation to normal organs such as liver, kidney, and bone marrow. There is a need to develop new strategies to increase the therapeutic index of cancer-targeting agents not only by enhancing rapid removal of the radioactivity from normal tissues but also improving the delivery of the radiopharmaceutical to the tumor. One strategy is to develop highly specific bio-degradable peptides to link the radio-metal chelate to the targeting agents. Our hypothesis is that bio-degradable linkers that can be cleaved on demand when incorporated in a tumor targeting agent such as monoclonal antibody, will result in an effective anti-cancer agent with high therapeutic index. A "one-bead one-compound" combinatorial peptide library method will be used to discover highly specific peptide substrates for tissue plasminogen activator (serine protease (TNKase(R), Activase(R))) to develop an "on-demand-cleavable" linker. In the past, we have shown that, when a cathepsin B sensitive linker was used in either of three radio-immunoconjugates, there was a 30-60% decrease in radiation dose to the liver but the radiation dose to the tumor was maintained. In this project, we have screened peptide libraries, with much higher diversity than the ones that were used in the past, for the discovery of highly specific peptide substrates for serine protease. We have explored the possibility of using TNKase (R) or Activase(R), FDA-approved thrombolytic agents, to cleave a linker between the radio-metal chelate and the antibody molecule so that free radio-metal chelate can be rapidly cleared through the kidney. Now identified, candidate peptides have been tested in vitro for specificity and efficiency and shown to be stable in plasma and media from malignant cells, yet readily clearable by low doses of TNKase. Peptides will be selected for final testing in mouse xenograft models. In year 3, a clinical protocol for evaluating the pharmacokinetics and efficacy of peptides with superior performance in the mouse model will be developed. This project interacts very closely with the other projects by providing them with novel bio-degradable linkers. This project also interacts very closely with the radiopharmacy core as well as the imaging and dosimetry core for both pre-clinical and clinical development.